Fluid-mold casting has been used for several years in connection with the production of ingot castings made of a number of different metals. In accordance with the process, a quantity of molten slag is placed at the bottom of an ingot mold and molten metal conditioned for the production of an ingot is teemed into the mold through the slag. During teeming, the slag advances upward on the surface of the metal and forms a thin coating on the ingot mold surface. The coating remains during the casting process and separates the ingot from the mold. When conditions in respect of metal and slag temperature melting point, and composition of the metal and the slag are compatible, an ingot is produced having a greatly improved surface as compared to that obtained when no casting slag is employed. The initial work conducted in accordance with the fluid-mold casting process involved the use of silicate type slags. These slags operated successfully in conjunction with the casting of metals such as mild steel and stainless steel. However, when it was attempted to use the silicate type slags with nickel and nickel-base alloys, it was found that numerous difficulties were encountered. For example, with many nickel alloys, there was an intolerable pick-up of silicon in the ingot resulting from interaction between molten metal and molten slag, yielding ingots which did not meet chemical specifications. In addition, defects were encountered in the surface of many ingots which have been classified as notch defect, a peripheral indentation about the ingot toward the toe portion, and as shotted-surface defect, which apparently involves emulsification of slag and metal and is usually most evident toward the top of the ingot. These defects required extensive and expensive overhaul of the ingots before further mill processing could be successfully undertaken. The result has been that the advantages contemplated through the use of the fluid-mold slag casting process, namely, improved ingot yield and better ingot surface, were not obtained in many instances. A further development in relation to slag chemistry involved the deletion of silica as a slag constituent and the use of a titania-calcium oxide-alumina type slag to provide an improved fluid-mold casting composition for use with nickel-containing alloys, particularly of the age hardening types. As another improvement a magnesia-calcium oxide-alumina slag was developed. Experience with these slag materials has demonstrated that even further improvement was necessary. For example, it was found that in the fluid-mold casting of nickel ingots intended for the production of wrought nickel products for electronic uses, there is an intolerable pick-up of titanium and aluminum from the titania-calcium oxide-alumina slag. This resulted in ingots which were chemically out of definition and which were not acceptable. When fluid-mold casting a Monel alloy with a magnesia-calcium-oxide-alumina calcium fluoride slag containing 7% magnesia, it was found that enough magnesium is picked up to impair hot malleability of the ingot. Furthermore, it was found that while in many instances highly satisfactory ingot surfaces were obtained in the production of nickel and nickel alloy ingots with the improved slags, in other instances unsatisfactory ingot surfaces, such as shotted surfaces, were still obtained. It was also found that during formation of the slag shell fractures occurred in the shell between the ingot surface and the mold wall, which allowed the molten metal to flow behind the slag shell, thus trapping flux on the ingot and resulting in the need for increased overhauling.
As noted above, many of the slags used heretofore for fluid-mold casting of metals contain as the predominant ingredients various combinations of oxides, e.g. lime and alumina plus silica, titania, or magnesia. In addition, they also contain fluorides such as cryolite (Na.sub.3 AlF.sub.6), and/or sodium fluoride (NaF) and/or calcium fluoride (CaF.sub.2). The purpose of the fluorides in these prior art slags is to adjust the melting point and control fluidity. Since the fluorides will attach refractory materials, carbon-lined furnaces are often used to heat the slags to the required temperatures, e.g. to about 3100.degree. to 3200.degree. F. In carbon-lined slag furnaces, however, the oxides react with the carbon lining, with the result that there is a foamy condition and high carbon pick-up in the slag. This tends to produce gassy nickel products.
It might also be noted that slags for fluid-mold casting cannot automatically be equated to casting slags for other processes such as electroslag refining or dry powder slag casting. In electroslag refining, for example, the electrical conductivity is a key feature of the slag, but it is not a factor in fluid-mold casting. On the other hand, fluid-mold slags must retain fluidity over a wide temperature range. For example, for fluid-mold casing nickel and nickel alloys it is highly desirable for the slag fluidity to be maintained over a range of at least about 3000.degree. down to about 2450.degree. F. Contrastingly, this temperature-fluidity requirement for the slag is not as critical in other ingot refining processes such as electroslag refining where a high slag temperature is maintained. For the dry powder-type slag, it is important that the slag have melting characteristics such that the dry slag in the mold can be converted to the molten condition by the molten metal per se, another characteristic not required of slags used for fluid-mold casting. Other characteristics of slags required for fluid-mold casting are that they must not foam either in the mold or on addition of the molten metal, and they must not react with the furnace or the molten metal in any way which would lead to undesirable inclusions or porosity in the ingot.
It has now been discovered that a special casting slag composition provides improved results in the fluid-mold casting of nickel, nickel-base and nickel-containing alloys, and nickel alloys containing age hardening elements, enables the production of sound, clean ingots of such alloys, and results in improved ingot surfaces and greater recovery of metal from the ingot into hot rolled products.
It is an object of the present invention to provide an improved fluid-mold casting slag particularly useful for the production of ingots made of nickel and nickel-containing alloys.
Another object of the invention is to provide a fluid-mold casting process applicable to nickel and nickel-containing alloys which provide improved ingot surface and improved metal yield upon hot rolling of the ingots, as well as improved metallurgical quality.
The invention also contemplates providing a casting slag composition useful for the production of ingots having improved surface quality in nickel-containing alloys which are age hardenable.